Courses
Courses for Kids
Free study material
Offline Centres
More
Store Icon
Store
seo-qna
SearchIcon
banner

A circle is inscribed in a triangle with sides 9, 12 and 15. The radius of the circle is

Answer
VerifiedVerified
533.7k+ views
1 likes
like imagedislike image
Hint: To find the radius of the incircle, first find the area of the triangle using the formula, 12×base×height
Here we have to find the radius of a circle inscribed in a triangle of sides 9, 12, 15.

Complete step-by-step answer:
The sides of the triangle given in the question are 9, 12, 15.
Let AB = 9, BC = 12, CA = 15.
Now we will check whether the given triangle is a right angled triangle. For this we will use Pythagoras theorem.
AB2+BC2=AC2
Substituting the corresponding values, we get
92+122=15281+144=225225=225
Hence, we can see that the given triangle satisfies Pythagoras theorem, so the given triangle is a right angled triangle. So, the corresponding diagram will be,
seo images

So, let the circle inscribed in the triangle ABC have the radius as ‘r’ and ‘O’ be the centre of the circle.
WE can see from the figure that the radius of the inscribed circle is perpendicular to the corresponding sides, so OD, OF, OE are perpendicular to AB, BC and AC respectively.
Now from figure, we can also say that
Area of triangle ABC = Area of triangle AOB + Area of triangle BOC + Area of triangle COA
Now we know the area of the triangle = ½ times base times height. So we can write it as,
ΔABC=ΔAOB+ΔBOC+ΔCOA12×AB×BC=12×OD×AB+12×OF×BC+12×OE×AC
Substituting values from the above figure, we get
12×9×12=12×r×9+12×r×12+12×r×15
Cancelling the like terms, we get
108=9r+12r+15r108=36rr=10836=3
Hence the radius of the inscribed circle is 3.

Note: Another approach for this problem is using the formula,
radius=(a+bc2)
Here a and b are the sides and c is the hypotenuse of the right angled triangle.
This is used when the circle is inscribed in a right angled triangle.
Latest Vedantu courses for you
Grade 10 | CBSE | SCHOOL | English
Vedantu 10 CBSE Pro Course - (2025-26)
calendar iconAcademic year 2025-26
language iconENGLISH
book iconUnlimited access till final school exam
tick
School Full course for CBSE students
PhysicsPhysics
Social scienceSocial science
ChemistryChemistry
MathsMaths
BiologyBiology
EnglishEnglish
₹41,000 (9% Off)
₹37,300 per year
Select and buy